Stock-Based Compensation Expense - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014 Employees	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of groups within the employee population	2
Percentage of stock options granted to all other employees expected to vest	70.00%
Percentage of stock options granted to senior management expected to vest	72.00%
Estimated forfeiture rate for unvested options for senior management	11.00%
Estimated forfeiture rate for unvested options for all non-senior management	12.00%
Increase In Compensation Expense Due To Revised Estimated Forfeiture Rates	$ 288,000	$ 300,000	$ 200,000
Weighted-average grant-date fair values of stock options granted	$ 15.97	$ 8.21	$ 6.58
Total intrinsic values of stock options exercised	31,884,000	32,096,000	16,296,000
Total fair values of stock options vested	11,627,000	9,717,000	9,362,000
Total unrecognized compensation expense related to non-vested stock options	19,389,000
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Period for Recognition	1 year 6 months 30 days
Stock-based compensation expense	15,158,000	10,620,000	8,520,000
Income tax benefit recognized related to stock-based compensation expense	4,977,000	3,482,000	2,772,000
Compensation expense capitalized	$ 0	$ 0	$ 0
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for grant under stock option plans	9,203,289
Vesting period for stock option plans	4 years
Expiration period of stock option plan	10 years
Expected dividend yield, beginning in 2015	0.56%